The thermal conductivity of glass is lower by roughly a factor of 2 to 3 than that of concrete or similar building materials. However, since, in most cases, panes are designed significantly thinner than comparable elements made of brick or concrete, buildings frequently lose the greatest share of heat via external glazing. This effect is particularly significant in high-rise buildings with partial or complete glass façades. The increased costs necessary for heating and air-conditioning systems make up a part of the maintenance costs of the building that must not be underestimated. Moreover, as a consequence of more stringent construction regulations, lower carbon dioxide emissions are required. An important approach to a solution for this involves triple-insulated glazing units, without which, primarily as a result of increasingly rapidly rising prices of raw materials and more stringent environmental protection constraints, it is no longer possible to imagine the building construction sector. Consequently, triple-insulated glazing units constitute an increasingly greater share of outward-directed glazing units.
Triple-insulated glazing units generally include three panes made of glass or polymeric materials that are separated from each other by two individual spacers. Another pane is placed on a double-glazing unit using an additional spacer. Very low tolerances apply during the assembly of such a triple glazing unit since the two spacers must be mounted at exactly the same height. Thus, the assembly of triple glazing units is substantially more complex compared to double glazing units because either additional system components must be provided for the assembly of another pane or a time-consuming multiple run through a conventional system is necessary.
The thermal insulation capacity of triple-insulated glass is, compared to single or double glazings, clearly higher. With special coatings, such as low-E coatings, this can be further increased and improved. So-called low-E coatings offer an effective capability of screening out infrared radiation already before entry into the living space and, at the same time, of letting daylight pass through. Low-E coatings are thermal radiation reflecting coatings that reflect a significant portion of the infrared radiation, which, in the summer, results in reduced warming of the living space. Extremely varied low-E coatings are, for example, known from DE 10 2009 006 062 A1, WO 2007/101964 A1, EP 0 912 455 B1, DE 199 27 683 C1, EP 1 218 307 B1, and EP 1 917 222 B1. Such low-E coatings cannot be applied to the center pane of a triple-glazing according to the prior art since the coating causes heating of the pane under sunlight that results in a failure of the adhesive bond between the center pane and the spacers. Moreover, adhesive bonding of the center pane to a functional coating generates additional stresses. To compensate the stresses, the center pane according to the prior art must be prestressed.